Heat Pump Electrical Supply Sizer
Air-Source & Ground-Source Circuit Specification
Required Overcurrent Protection
Type C MCB or RCBO Recommended
Specifying Heat Pump Electrical Supplies
Air Source Heat Pumps (ASHP) and Ground Source Heat Pumps (GSHP) represent a significant continuous electrical load for domestic and commercial properties. Unlike standard heating elements, heat pumps utilise compressor motors governed by inverter drives, meaning their electrical characteristics demand precise engineering to comply with BS 7671:2018+A3:2024.
1. Thermal Output vs. Electrical Input (COP)
A frequent error made by installers is confusing the thermal rating with the electrical rating. A "12kW Heat Pump" does not consume 12kW of electricity (which would be over 50 Amps!). Heat pumps capture ambient heat from the air or ground.
The Coefficient of Performance (COP) defines the efficiency. If a 12kW unit operates at a COP of 3.0, it produces 3kW of heat for every 1kW of electricity consumed. Therefore, the actual electrical input to the compressor is only 4kW (approx. 17 Amps). This baseline must then be combined with any integrated backup immersion heaters.
2. Auxiliary Immersion Heaters
During extreme winter temperatures where the COP plummets, or during mandatory high-temperature legionella sterilization cycles, the heat pump relies on a standard resistive immersion heater (usually 3kW) located in the cylinder. This load is additive. A 4kW compressor plus a 3kW backup heater results in a peak total demand of 7kW (approx. 30.4 Amps). The dedicated radial circuit must be sized for this absolute maximum combined load.
3. MCB Types and Inrush Currents
Compressor motors generate an "inrush" or "locked rotor" current when starting up, which can briefly spike to multiple times the running current. While modern variable-speed inverter drives smooth this out considerably, standard practice dictates the use of a Type C MCB or RCBO. A Type B device may suffer from nuisance tripping over time due to motor startup characteristics.
4. RCD Specification for Inverter Drives
Heat pumps alter alternating current (AC) into direct current (DC) and back again to control motor speed. In the event of a fault, variable frequency drives can leak high-frequency AC or smooth DC fault currents back into the installation. A standard Type AC RCD will be blinded by this. You must refer to manufacturer instructions, but heat pumps generally require at minimum a Type A or ideally a Type F RCD to guarantee life safety.